CharSequenceDemo in the Sun Java Tutorials

Question

Having spent the last 30 years doing Prime Information/Infobasic, I'm trying to get my head around the basics of Java.
I need help with interpreting what is going on in this method, from CharSequenceDemo.java: (my line numbers)
1. public CharSequence subSequence(int start, int end) {
2. if (start < 0) {
3. throw new StringIndexOutOfBoundsException(start);
4. }
5. if (end > s.length()) {
6. throw new StringIndexOutOfBoundsException(end);
7. }
8. if (start > end) {
9. throw new StringIndexOutOfBoundsException(start - end);
10. }
11. StringBuilder sub =
12. new StringBuilder(s.subSequence(fromEnd(end), fromEnd(start)));
13. return sub.reverse();
14. }
To my, perhaps old-fashioned, brain, it looks as though line 12 invokes the method containing it, since string s is defined in main() as a a CharSequenceDemo type, and subSequence is this very same method within its instantiating class (CharSequenceDemo), and is therefore a recursive method call. s.subSequence(fromEnd(end), fromEnd(start)) (in line 12) is a parameter which must be evaluated for use in the StringBuilder constructor, surely? But when I run the program with debug statements, its obvious that there's nothing recursive about it.
Please can someone help sort out my misunderstanding of what's going on here?
The whole class is:
// CharSequenceDemo presents a String value -- backwards.
public class CharSequenceDemo implements CharSequence {
private String s;
public CharSequenceDemo(String s) {
//It would be much more efficient to just reverse the string
//in the constructor. But a lot less fun!
this.s = s;
//If the string is backwards, the end is the beginning!
private int fromEnd(int i) {
return s.length() - 1 - i;
public char charAt(int i) {
if ((i < 0) || (i >= s.length())) {
throw new StringIndexOutOfBoundsException(i);
return s.charAt(fromEnd(i));
public int length() {
return s.length();
public CharSequence subSequence(int start, int end) {
if (start < 0) {
throw new StringIndexOutOfBoundsException(start);
if (end > s.length()) {
throw new StringIndexOutOfBoundsException(end);
if (start > end) {
throw new StringIndexOutOfBoundsException(start - end);
StringBuilder sub =
new StringBuilder(s.subSequence(fromEnd(end), fromEnd(start)));
return sub.reverse();
public String toString() {
StringBuilder s = new StringBuilder(this.s);
return s.reverse().toString();
//Random int from 0 to max.
private static int random(int max) {
return (int) Math.round(Math.random() * max + 0.5);
public static void main(String[] args) {
CharSequenceDemo s =
new CharSequenceDemo("Write a class that implements the CharSequence interface found in the java.lang package.");
//exercise charAt() and length()
for (int i = 0; i < s.length(); i++) {
System.out.println(s.charAt(i));
//exercise subSequence() and length();
int start = random(s.length() - 1);
int end = random(s.length() - 1 - start) + start;
System.out.println(s.subSequence(start, end));
//exercise toString();
System.out.println(s);
}

Answer 1

It's hard to read code that isn't formatted. Please press the code button and paste the code between the generated tags next time you post code.
It looks like there one subSequence method in the CharSequenceDemo class, but there's also a subSequence method in the String class.
s.subSequence(....)"s" is a reference to a String, so that invokes subSequence in the String class.
Kaj